As well-known in the art, a Doherty power amplifier has a structure of a carrier and a peaking amplifier connected in parallel by using a quarter-wave transformer (λ/4 line). Further, the Doherty amplifier is driven by a symmetrical power driving method in which the peaking amplifier controls a load impedance of the carrier amplifier by increasing the amount of current supplied to the load from the peaking amplifier as the power level is increased, thereby improving efficiency.
A microwave Doherty amplifier has been used in an amplitude modulation (AM) transmitter of a broadcasting apparatus using a high-power low-frequency (LF) vacuum tube or a medium frequency (MF) vacuum tube. There have been a variety of proposals for implementing the microwave Doherty amplifier with a solid-state device without using a vacuum tube, and numerous researches have been conducted to implement the proposals.
A Doherty amplifier employing an asymmetric power driving method has achieved high efficiency and linearity. Especially, the Doherty amplifier employed in base stations and handsets for mobile communications is implemented by using approximately same-sized solid-state devices, the same input and output matching circuits and an input power drive. In this case, a carrier amplifier is biased as a class AB, and a peaking amplifier a class C. Since the peaking amplifier has a lower bias than that of the carrier amplifier, it is problematic that the current level of the peaking amplifier is always lower than that of the carrier amplifier depending on a power level.
FIG. 1 shows the amplitude of a current component according to each bias level, i.e., a conduction angle. As illustrated in FIG. 1, a fundamental current component's amplitude of the peaking amplifier biased at a lower level is lower than that of the carrier amplifier. The carrier amplifier whose bias point is the class AB has a conduction angle ranging from π to 2π and, therefore, has the amplitude of the fundamental current component ranging from 0.5 to 0.536 at a maximum input power. Meanwhile, the peaking amplifier operated in the class C has a conduction angle ranging from 0 to π and, thus, has the amplitude of the fundamental current component ranging from 0 to 0.5. Accordingly, the fundamental current component of the peaking amplifier does not reach that of the carrier amplifier. As a result, a load modulation (a phenomenon in which load impedance of the front end of a current source varies with an amplitude of current generated from the current source) occurs and, further, serious problems are caused in the Doherty operation.
Moreover, as shown in FIG. 9A to be described later, due to the lower bias level of the peaking amplifier, the fundamental current component of the peaking amplifier is detected only when a driving voltage thereof is equal to or higher than a certain level.
Therefore, if the carrier amplifier is driven at a maximum input power, the fundamental current component level of the peaking amplifier is lower than that of the carrier amplifier and, further, the peaking amplifier is not driven at the required maximum input power, thereby generating a much lower fundamental current. As a result, the Doherty amplifier is not able to generate a desired maximum output power.
In order to overcome the above-described problems, a research finding has been published regarding considerably improved a typical Doherty amplifier in terms of a maximum output power while maintaining the high efficiency thereof by employing an envelope tracking device or an input power tracking device. Further, various researches have been done in order to actually implement the Doherty amplifier in a microwave bandwidth, and one of them is shown in FIG. 2.
A Doherty amplifier shown in FIG. 2 includes a carrier amplifier 204 and a peaking amplifier 206 in parallel; a power divider 200 for providing a same power to the carrier amplifier 204 and the peaking amplifier 206; a transmission line 202 for synchronizing phases between the carrier amplifier 204 and the peaking amplifier 206; an offset line 208 for generating a proper load modulation by increasing an impedance output while the peaking amplifier 206 is not operating; and quarter-wave transmission lines 210 and 212 for performing the Doherty operation.
Such Doherty amplifier as in FIG. 2 is implemented in a manner that the carrier amplifier and the peaking amplifier have the same input/output matching circuits and yields the same output, so that a maximum output can be produced by each Doherty amplifier. Further, by sequentially providing the output matching circuits and, in turn, the offset line 208 at output ends of transistors in the carrier and the peaking amplifiers, an imaginary part as well as a real part can be matched, thereby enabling the Doherty operation while obtaining the maximum output power. (See, Y. Yang et al, “Optimum Design for Linearity and Efficiency of Microwave Doherty Amplifier Using a New Load Matching Technique,” Microwave Journal, Vol 44, No. 12, pp. 20-36, December 2002.)
Further, FIG. 3 provides an N-way Doherty amplifier having an optimum design for efficiency and linearity while improving a typical Doherty amplifier (See, Y. Yang et al, “A Fully Matched N-way Doherty Amplifier with Optimized Linearity,” IEEE Trans. Microwave Theory and Tech., Vol. 51, No. 3, pp. 986-993, March 2003.)
Unlike the configuration shown in FIG. 2, the N-way Doherty amplifier illustrated in FIG. 3 performs the Doherty operation with a single carrier amplifier 302 and a (N−1)-number of peaking amplifiers 304. Further, an N-way splitter 300 is used for putting the same input into the single carrier amplifier 302 and the peaking amplifiers 304.
FIG. 4 presents an N-stage Doherty amplifier for gradually achieving high efficiency from a much lower power level in comparison with a general Doherty amplifier (See, N. Srirattana et al, “Analysis and design of a high efficiency multistage Doherty amplifier for WCDMA,” EuMC Digest 2003, Vol. 3, pp. 1337-1340, October 2003.) The Doherty amplifier shown in FIG. 4 has an N-way power divider 400 for performing the Doherty operation by putting the same input into a single carrier amplifier 402 and a (N−1)-number of peaking amplifiers 404.
The following is a brief explanation of the Doherty amplifier. First of all, the carrier amplifier 402 is turned on and, then, a first peaking amplifier PA1 is turned on, to perform the Doherty operation. Thereafter, both of the carrier amplifier 402 and the first peaking amplifier PA1 serve as a carrier amplifier while a second peaking amplifier PA2 serves as a peaking amplifier, thereby performing the Doherty operation together. Such operation is carried out up to a last peaking amplifier PANN−1. As the Doherty operation is performed gradually and successively as described above, maximum efficiency can be obtained from a much lower power level. Further, it is also possible to repeatedly obtain the maximum efficiency over intermediate power levels, so that high efficiency can be obtained over a full power level.
In the meantime, in order to solve a problem in which the Doherty amplifier do not produce a maximum power output due to a low bias when the Doherty amplifier is implemented by using a solid-state device, there has been proposed a Doherty amplifier by using an envelope tracking device (See, Y. Yang et al, “A Microwave Doherty Amplifier Employing Envelope Tracking Technique for High Efficiency and Linearity”, IEEE Microwave and Wireless Components Letters, Vol. 13, No. 9, September 2003., and J. Cha et al, “An Adaptive Bias Controlled Power Amplifier with a Load-Modulated Combining Scheme for High Efficiency and Linearity”, IEEE MTT-S Int. Microwave Sympo. Vol. 1, pp. 81-84, June 2003.) However, even in the proposed Doherty amplifier, an additional device for controlling the power level of the peaking and the carrier power amplifiers is still required in order to achieve an improved linearity and the maximum output.